Threaded joint for an oil-well tubing

ABSTRACT

The engineering problem of the claimed invention consists in the increase of operating reliability of the threaded joint, whose coupling has the coating applied by the diffusion galvanizing method, by way of ensuring a proper standard tightness. The technical result of the increase of the inlet cylindrical counterbore length in the internally threaded element by the value of the thread tightness increment caused by galvanizing and determined from the claimed empirical formula consists in increased reliability of the joint under service conditions as the probability of overtightening the joint for the sake of attaining the nominal tightness, as well as damage to the end thread turns projected from the coupling on service are ruled out. This result is attained without consumption of time and funds for re-training of the personnel, publication and delivery of new normative materials to each drilling site, manufacture of new calibrating tools and repair mountings. In its turn, this makes it possible to immediately begin supplying the suggested joints and increase the service life of standard threaded joints of oil-well tubings from 6÷8 to at least 50 screwing/unscrewing cycles. Besides, in case of forced screwing with a standard torque the joint reliability also increases because the threaded part of the externally threaded element is nearly completely hidden inside the counterbore due to the increased counterbore length and thus protected against the mechanical effect of the well medium.

RELATED APPLICATIONS

This application is a Continuation of PCT application serial numberPCT/RU2007/000529 filed on Oct. 3, 2007, which in turn claims priorityto Russian Patent Application No. 2006135967 filed on Oct. 11, 2006,both of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention proposal relates to tapered threaded joints mainlyutilized in drilling equipment and more particularly to threadedconnection assemblies of oil-well tubing or drill tubes in expendable,oil, or gas wells.

BACKGROUND OF THE INVENTION

The terms and expressions used below have the following interpretations:

“Joint or tubing joint” is the knockdown assembly comprising theexternally threaded element, for example, tube or reducing couplingsometimes called “union nipple” in the standard materials, and theinternally threaded element, for example, coupling or reducing couplinghereinafter referred to as “coupling”.

“Axial thread tightness” or “tightness” is the value measured by thedistance between the plane passing through the thread runout end on thenipple and the plane of the coupling face (GOST 633-80). The tightnessis positive if the thread end is outside the coupling, zero if thethread end and coupling face coincide, and negative if the thread end isinside the coupling. According to the same GOST, to check the threadusability, the tightness value is measured in the joint coupled fingertight.

“Thread runout end” or “thread end” (according to GOST 633-80) is thecrosspoint of the vanish cone generator with the generator of thecylinder, whose diameter is equal to the external diameter of the nipple(tube).

Other terms used in the present description: thread pitch, thread, etc.relate to threaded joints of oil-well tubings (hereinafter referred toas OWT).

Oil-well tubings are operated under heavy conditions of impact andsign-changing loads, under high pressure, often at the increasedtemperature and in aggressive media. At that the threaded joints of suchtubings should be reliable and air-tight at pressures of up to thousandsof atmospheres. Irrespective of the use of sealing and thread lockcompounds of various kinds, application of high tightening forces closeto the limit, past which thread stripping occurs, remains the basicmethod of ensuring reliability and tightness of the joints. Owing tothis the service life of the threaded elements does not exceed butseveral screwing/unscrewing cycles.

Known is the tapered threaded joint of the oil-well tubing comprising atubing joint that has a cylindrical boring and a mated nipple, forexample, tube or reducing coupling (GOST 633-80, Dwg 6). According toabove GOST the coupling thread should have a zinc or phosphate coating.The standard does not regulate the method of coating application, but atthe time of its development use was made of only electrolytic coatingswith a thickness in the order of 10 μs, which were comparatively softand non-wear-resistant and intended for thread protection againstcorrosion during storage and transportation of threaded elements. Thehot zinc plating method by dipping a coupling with liquefied zinc is notapplied due to irregularity of the coating thickness on the thread andzinc sags on the thread hindering the check with gages. The electrolyticzinc coating features low mechanical strength and are susceptible tohydrogen embrittlement under the conditions of the corrosive environmentin an oil or gas well. It is more expensive than the phosphate one andat present is used only on the threads of casing tubes, where just oneor two screwings with subsequent casing cementing. At present allRussian tube-rolling mills employ only phosphate coating of thecouplings of oil-well tubing. By virtue of the properties of abovecoatings the standard requirements to provision of threaded jointtightnesses are based on the most widely spread phosphate coating of thecoupling thread in the order of 10 μs.

The shortcoming of the known joint consists in its low service life.Thus, in “Operating Instructions for Oil-Well Tubing” RD 39-136-95,Para. 7.15 states: “The number of screwing/unscrewing cycles of thethreaded joints should be recorded in the course of operation ofoil-well tubing. According to the performed studies durability of thethreaded joints is retained for up to 6-8 cycles”. This is a very smallvalue if one takes into account the fact that even at the operated wellsthe tubes are elevated for cleaning or replacement of the pumpingequipment. Therefore, the main task of the developers consists in theincrease of the durability of the oil-well tubing joints.

At a zero tightness, when the external thread is manually screwed in thecoupling to the last thread turn, the joint is rejected. The joint wearis often manifested not in the reduced profile of the thread turns withthe tightness reduction, but in the form of increased tightness inexcess of the above value because of appearance of scorings on thecontacting surfaces. If tightness exceeds the standard value, the jointis also rejected.

Known is the threaded element for the tube threaded joint with a highlimit endurance, in which at least some threads contain a spiral groovereaching the crest (RF Patent No. 2261395 according to class F16L15/06).Owing to pliability of the threads, which are weakened at the crest bythe groove, more even load distribution among the thread turns, i.e.,the joint reliability is increased. The shortcoming of the known jointconsists in the difficulty-to-make of its elements. Besides, such athread is easily galled in case of casual handling. Restoration ofworn-out threaded elements is possible only under the conditions of thespecialized repair centers, which cannot be numerous due to complexityof equipment. Another shortcoming of the known threaded element is itsirregularity. The oil-well tubings, as well as the elements for theirjointing are the vital assemblies of the drilling technique and arewidely employed at the vastest territories. Therefore, they should bestrictly standardized and unified for ensuring both reliability andcompatibility. Development of new kinds of joints with improvedperformance characteristics is obviously necessary, but their check andintroduction require many years of work. Therefore, the technicalconcepts improving the service life of standardized threaded jointswithout deviation from the requirements of the standards are morerelevant and called for.

Known is the threaded joint for tubes comprising threaded sections andtightly fitted unthreaded sections that ensure air tightness whentightening the thread (RF Patent No. 2258171 according to classF16L15/00). Separation of the sections ensuring air tightness and thoseensuring the required compacting force is the effective technique widelyemployed in vacuum engineering. However, with respect to the drillingequipment this technique does not solve the problem of increasing thethread service life as the tightening forces still remain huge and thejoint does not withstand more than few cycles without repair. Besides,its air tightness greatly depends on the fineness of mated surfaces,which cannot be always ensured afield. Another shortcoming of the knownjoint is the fact that it requires higher accuracy of manufacturing,which in its turn aggravated by the location of one of the unthreadedsections ensuring air tightness deep inside the internally threadedelement (coupling). One more shortcoming of the known joint is the factthat its geometry does not comply with the effective standards.

Also known is the threaded joint for steel tubes comprising threadedsections and tightly fitted unthreaded sections that ensure airtightness when tightening the thread (RF Patent No. 2248495 according toclass F16L15/04). Solid-film lubricant with a porous-zinc or zinc-alloysublayer is applied onto at least one of the sections by blasting withparticles consisting of iron cores and zinc shells. Such a joint ensuresmore reliable air tightness of the joint than the preceding analog, butthis is attained through considerable complication of the manufacture,the more so that one of unthreaded sections ensuring air tightness islocated deep inside the internally threaded element (coupling), whichhampers its processing. The remaining shortcomings of the known jointare similar to those of the preceding analog.

Also known are the threaded joints for steel tubes, in which solid-filmlubricant consisting of a lubrication powder (for example, molybdenumdisulfide) and a binding agent is applied onto the threaded surfaces ofat least one of the sections for increasing their reliability (RFPatents Nos 2258170, 2258859, and 2262029 according to class F16L15/00).The shortcoming of the known joints consists in complicacy ofpreparation and application of solid-film lubricant, which requiresspecial preparatory treatment of the threaded surface, includingcreation of the porous sublayer. Irrespective of all complicacy, theservice life of these joints does not exceed 10 to 20screwing/unscrewing cycles.

Known is the method of manufacture of the threaded joint with atrapezoid profile of the tapered thread for the oil well tubing. Thismethod consists in primarily setting seven basic parameters externaldiameter and thickness of the tube, dimensions of the tight spigot,etc.) and then in determining seven dependent parameters (length andtaper of the seal, shoulder angle, flank angle, etc.) from the suggestedformulas (RF application for invention No. 2003130748 according to classF16L15/04). The published patent claim does not contain the results ofpractical implementation of the suggested method, which would enable tojudge its efficiency. Besides, the calculated parameters are alwayscorrected in practice pursuant to the test results as the dimensions ofthe finished articles inevitably have deviations from those preset onthe basis of computations. As the joint designed in accordance with theknown method will not correspond to the effective standards, itsmastering requires long time.

Known is the tube of the tubing string comprising a joint coupling andreducing coupling, whose threaded surfaces are covered with a two-layerprotecting coating consisting a zinc layer, 10 to 14 μm thick, appliedby the thermal diffusion galvanizing method and a phosphatic film, 2 to3 μm thick, applied onto the former (RF Utility Patent No. 38498according to class F16L15/08). The suggested coating ensuresanti-corrosion protection of the joint elements during storage andtransportation, but the coating thickness is insufficient for reliablehermetic encapsulation of the joint and increase of its service life.The phosphatic coating is undurable and easily eroded. As in threadingthe surface finish is usually fixed at Rz 20, the declared thickness of2 to 3 μm is several times smaller than unevennesses and by no means canconsiderably increase the thread service life. The technology of thermaldiffusion galvanizing of the thread surfaces corresponding to the abovesurface finish does not ensure guaranteed continuity of the coating,less than 15 μm thick.

The taper threaded joint of oil-well tubing or tubing string with thejoint coupling featuring the inlet cylindrical counterbore and the matedexternally threaded element (nipple), example, tube or reducingcoupling, where thermal diffusion powder galvanizing is made on thethreaded surfaces of the coupling and externally threaded element is theclosest to the suggested one by the technical essence and attainableresult. (RF Utility Patent No. 30913 according to class F16L15/08). Thethickness of the coating on the threaded surfaces is selectedexperimentally so that the coating is entire and rather strong and atthe same time does not peel in screwing. The aim of the coatingapplication is to increase the joint service life rather thananti-corrosion protection and the coating is quite thicker than thoseusually employed. Therefore, the coating noticeably increases the jointtightness. Nearly half of the manufactured couplings satisfying thestandard requirements prior to the coating application had the tightnessin excess of the permissible limits after the coating application. Thiscompels the user to sort out the manufactured couplings prior to thecoating application so that only a thin phosphatic coating not ensuringthe service life increase is applied on those, which will not satisfythe standard requirements after galvanizing. If the user ordered onlygalvanized couplings with the increased service life, the output offinished articles evaluated with respect to the tightness value will benearly the half of the production which unacceptable for themanufacture, particularly in case of making the couplings of expensivesteel with special properties.

Under service conditions the joint aptness is evaluated with respect tothe tightness value in the manually screwed joint. According to above RD39-136-95 “ . . . if the nipple thread with a torque being smaller thanthe minimum one is screwed into the coupling to the last thread turn orif after screwing with the maximum torque two vacant turns have notentered into the coupling, both tubes shall be rejected . . . ”. Thetightness is visually evaluated by workmen proceeding from the number ofturns projecting over the coupling edge and the screwing torque of thehydraulic wrench. When new couplings with thermal diffusion galvanizingand an increased tightness are delivered to the wells, the workmenguided by the instructions and previous experience will either rejectnew couplings or try to attain the required tightness by exceeding thestandard screwing torque, thus reducing the joint reliability.Therefore, the known solution, which is rather effective, as has beenproved by the tests, turn out to be inapplicable in practice on a widescale due to the necessity to change the instructions, retraining of thepersonnel, manufacture of new calibration tools, repair mountings andall these should be done at thousands of wells all over the world.

The known coupling has one more shortcoming. According to the effectivestandards after forced screwing there should remain not more than twonipple turns, while in the known joint more than two thread turns remainoutside the coupling in case of forced screwing. During benchmark testsfor service life evaluated by the number of screwing/unscrewing cyclesit is of no importance, but under service conditions the turnsunprotected by the coupling counterbore are damaged by the well medium'smechanical effect and can be worn out or damaged even prior to the firstelevation from the well. As far as they wear out, the tightness value isreduced, the turns go deeper into the coupling and the joint has to berejected when the mutilated nipple turns reach the coupling turns thoughthis joint has not worked out the service life determined by thebenchmark tests. For ordinary joints withstanding 6-8 cycles the wear ordamage of the turns projecting over is not so probable and high unlikein the case of the joints' coatings withstanding over 50 cycles.Therefore, the service life increase of the known joint cannot beguaranteed even if the above organizational restrictions can beovercome.

SUMMARY OF THE INVENTION

The technical problem solved by the present invention proposal consistsin increasing operational reliability of the threaded joint, whosecoupling has a coating on the threaded part applied by the thermaldiffusion galvanizing method by means of a proper standard tightness.

The above problem is solved when in the known tubular joint coupling ofthe oil-well tubing or drill string comprising a threaded element withexternal taper triangular thread, for example, tube or reducingcoupling, and a threaded element with internal thread, for example,coupling or reducing coupling, having the inlet cylindrical counterboreand the threaded part with a coating applied by the thermal diffusiongalvanizing method the length of the inlet cylindrical counterbore ofthe coupling is increased by the value of the increment of threadtightness ΔA calculated from the formula: ΔA=kδ_(min)÷kδ_(max) where kis the empirical coefficient equal to 70 and δ_(min) and δ_(max) are theminimum and maximum coating thicknesses, respectively.

The above problem is also solved when in the known tubular jointcoupling of the oil-well tubing or drill string comprising a threadedelement with external taper trapezoid thread, for example, tube orreducing coupling, and a threaded element with internal thread, forexample, coupling or reducing coupling, having the inlet cylindricalcounterbore and the threaded part with a coating applied by the thermaldiffusion powder galvanizing method the length of the inlet cylindricalcounterbore of the coupling is increased by the value of the incrementof thread tightness ΔA calculated from the formula:ΔA=kδ_(min)÷kδ_(max), where k is the empirical coefficient equal to 30and δ_(min) and δ_(max) are the minimum and maximum coating thicknesses,respectively.

The technical result of extension of the length of the inlet cylindricalcounterbore of the internally threaded coupling by the tightnessincrement value caused by galvanizing, which is determined from thedeclared empirical formula consists in increasing the joint reliabilityunder service conditions, as a possibility of overtightening the jointin the desire to reach the nominal tightness and a damage of end threadturns projecting from the coupling are ruled out in the course ofoperation. This result is attained without consumption of time and fundsfor re-training of the personnel, publication and delivery of newnormative materials to each drilling site, manufacture of newcalibrating tools and repair mountings. In its turn, this makes itpossible to immediately begin supplying the suggested joints andincrease the service life of standard threaded joints of oil-welltubings from 6÷8 to at least 50 screwing/unscrewing cycles.

Besides, in case of forced screwing with a standard torque the jointreliability also increases because the threaded part of the externallythreaded element is nearly completely hidden inside the counterbore dueto the increased counterbore length and thus protected against themechanical effect of the well medium.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a view of one manually screwed threaded joint, whose couplinghas a coating on a threaded part.

FIG. 2 is a view of a suggested manually screwed threaded joint.

FIG. 3 is a view of a suggested threaded joint screwed with anestablished operating torque.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The suggested threaded joint (FIG. 1) consists of the externallythreaded element 1, for example, a well tube or a reducing coupling,hereinafter for brevity referred to as the nipple, and the internallythreaded element 2 mated with it, for example, a coupling or a reducingcoupling, hereinafter for brevity referred to as the coupling. The entrysection of the coupling 2 has the cylindrical counterbore 3. Thecounterbore diameter is somewhat greater than the diameter of theunthreaded part of the nipple 1. The coating 4 applied by the diffusionpowder galvanizing method is found on the threaded part of the coupling.The thickness of the coating 4 is usually within 15 to 30 μm, but can beeven thicker—up to 50 μm. An anti-corrosive phosphatic coating with athickness of several microns can be applied over the galvanized coating.The threaded surface of the nipple 1 has no coating or thestandard-corrosive phosphatic coating with a thickness of 2 to 3 μm isapplied onto it.

Depth G of the counterbore 3 measured from the face end 5 of thecoupling 2 to the thread beginning is equal to the sum of standard depthg of the counterbore rated for joints without galvanized coating and thevalue of thread tightness increment ΔA, i.e., G=g+ΔA. At that the valueof ΔA determined experimentally does not depend on the thread pitch ordiameter and is calculated from the formula: ΔA=kδ_(min)÷kδ_(max) wherek is the empirical coefficient equal to 70 for triangular thread and 30for trapezoid thread for the coating thickness within 15 to 50 μm. Indetermining the formula for ΔA, it was taken into account that the wellattending personnel determine the tightness by sight according to thenumber of turns projecting over the coupling edge. Accuracy of suchdetermination is equal to approximately ¼÷⅓ of a turn and, accordingly,the suggested couplings, whose tightness is within the standardtolerance zone, are found usable under service conditions no matterwhether the counterbore depth is increased by kδ_(min) or by kδ_(max).

The expression for ΔA is true for any standard taper threadsirrespective of the thread diameter or pitch. The values of k aredetermined experimentally during the tests of the coated couplings.Setting the value of ΔA proceeding from mean coating thickness δ_(med)established due to technological considerations as optimal for the jointservice life and coating strength is preferable. In this case the valueof ΔA will be equal to kδ_(med). The value of δ_(med) need not be themean arithmetic value between δ_(min) and δ_(max). Thus, it has beenexperimentally established that the increased service life of the jointis ensured at the coating thicknesses being within a range of 15 to 30μm, but the best results are attained if the parameters of theengineering process of coating application are oriented towardsobtaining δ_(med)=25 μm. However, if due to the engineering processdeviations the mean coating thickness in one of the batches of couplingsappears to be less than δ_(med), but within the tolerance zone, it isexpedient to calculate a new value of ΔA corresponding to this batch andaccordingly reduce the counterbore length. As long as the coupling hasthreads and counterbores on both sides, the coupling length is increasedby 2ΔA as a result of the use of the suggested solution. As for thereducing couplings having the internal thread only at one side, theirlength is increased by ΔA.

Different values of coefficients k for triangular and trapezoid threadsis explained by the fact that in the triangular thread the turns of thenipple and coupling contact along the thread centerline, i.e., along theflanks, while in the joints with the trapezoid thread the fit is donewith respect to the internal or internal and external thread diameters.

The suggested joint is manufactured and used as follows.

The coupling blank is made with the counterbore depth increased by avalue of ΔA. The increase of the externally threaded element by a valueof ΔA for reducing couplings and by a value of 2 ΔA for couplings ispreferable. In the coupling thread-cutting machine compensation by avalue of ΔA is introduced so that the basic thread plane displace by thesame distance towards the smaller diameter of the thread taper. Afterthread cutting in the coupling the axial tightness is checked by meansof a standard thread gage (FIG. 2). At that, the couplings with thetightness equal to tightness A preset in the standards for threadedjoints less the value of ΔA, i.e. the couplings with tightness (A−ΔA)are considered fit. Thus the whole tightness tolerance zone preset inthe standards is implemented. As long as the departure from the standardinspection method takes place at the intermediate manufacture stage, nocoordination or permits are required.

The couplings that have passed the test are subjected to diffusionpowder galvanizing. The thickness of the applied coating can amount to15 to 50 μm. The manufactured couplings are subjected to outgoinginspection by means of standard gages in accordance with the standardprocedure, i.e., the coupling tightness checked with the aid of a pluggage should amount to standard value A with allowance for limitdeviations. In this case, the incoming inspection of couplings at theuser's will indicate standard value A, i.e., any changes in theoperating instructions will not be needed.

During the first forced screwing of the suggested joint the tightnessvalue is reduced by 0.5 to 0.8 mm. Further on the tightness of thegalvanized coupling's thread is reduced at an average rate of 0.05 mmper screwing/unscrewing cycle.

The experience of benchmark and field tests of oil-well tubings withcouplings, whose thread is galvanized by the thermal diffusion methodand the counterbore length is increased according to the presentsuggestion shows that these joints are assembled with the torquessimilar to ordinary oil-well tubings. Axial and diametral tightnesses ofthe joint also correspond to those of ordinary oil-well tubings. Inassembling the joints use was made of greases usually employed in theoilmen's practice. Assembly of the suggested joints does not require anyspecial equipment. There were no cases of rejection of fit joints by thewell attending personnel.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A threaded joint for an oil-well tubing comprising: a threadedelement with external tapered triangular threading; and a threadedelement with internal threading including: a cylindrical recess inlet,and a threaded portion, wherein the threaded portion has at least acoating applied by a thermal diffusion powder galvanizing method,wherein the length of the cylindrical recess inlet is increased by anincrement value of tightness caused by the coating, and wherein theincrement value ΔA is calculated in accordance with the followingformula:ΔA=kδ _(min) ÷kδ _(max), wherein k is an empirical coefficient equal to70, and δ_(min) and δ_(max) are minimum and maximum coating thicknesses,respectively.
 2. A threaded joint for an oil-well tubing comprising: athreaded element with external tapered trapezoid threading; and athreaded element with internal threading including: a cylindrical recessinlet, and a threaded portion, wherein the threaded portion has at leasta coating applied by a thermal diffusion powder galvanizing method,wherein the length of the cylindrical recess inlet is increased by anincrement value of tightness caused by the coating, and wherein theincrement value ΔA is calculated in accordance with the followingformula:ΔA=kδ_(min)÷kδ_(max), wherein k is an empirical coefficient equal to 30,and δ_(min) and δ_(max) are minimum and maximum coating thicknesses,respectively.